Relay allotter for finder switches



July 22, 1947.

Filed Dec. 8, 1944 J. w. M cLEw ET AL RELAY ALLOTTER FOR FINDER SWITCHES 6 Sheets-Sheet 2 OWEN A. PEARCE ATTORNEY 6 Sheets-Sheet 6 UF-OF UTA OTA AEU 3 INVENTORS JOHN W. Mc CLEW OWEN A. PEARCE 3%414 ATTORNEY July 22, 1947.

J. w. M CLEW ET AL RELAY ALLOTTER FOR FINDER SWITCHES Filed Dec 8, 1944 UF-OF v vq GRR UTA UTB AEU Z Patented July 22, 1947 RELAY ALLOTTER FOR FINDER SWITCHES John William McClew and Owen Avis Pearce, Liverpool, England, assignors to Automatic Electric Laboratories Inc., Chicago, 111., a corporation of Delaware Application December 8, 1944, Serial No. 567,223 In Great Britain January 22, 1944 6 Claims.

The present invention relates to telephone or like systems and is more particularly concerned with finder switch allotting arrangements such as find particular use in automatic telephone systems of the line finder type.

In finder switch main exchanges the subscribers lines are usually divided up into 100 or 200- line groups, each such group being served by a predetermined number of two-motion line finder switches which are allotted for use by a uni-directional switch, for instance in the manner disclosed in British Patent No. 462,910, issued June 10, 1937.

In private automatic exchanges or private automatic branch exchanges of the size normally met with, a number of finder/connector link circuits usually serve the subscribers lines concerned, the links being allotted for use by associated relays operating on a chain circuit basis for instance as described in British Patent No. 389,867, issued June 15, 1933.

A primary object of the present invention is to combine the cyclic allotting capabilities of the selecting switch allotter with the substantially instantaneous action of the relay type allotter.

According to one feature of the invention, in a telephone or like system having a group of lines served by a plurality of finder switches, an idle finder switch for use in handlin a call is allotted under the control of a relay allotter incorporating a closed chain circuit having a relay connected thereto corresponding to each finder and a one-way conducting device in the chain between each relay connection and the next together with a switching device for applying starting potential to each relay connection in the chain in turn.

According to another feature of the invention, in a telephone or like system having a group of lines served by a plurality of finder switches certain of the lines are given calling priority by means of arrangements adapted to remove any previously existing markings from the banks when a privileged line calls so that the next succeeding operation of a finder switch will connect with the privileged line.

The invention will be better understood from the following description of one method of carrying it into effect, reference being had to the accompanying drawings comprising Figs. 1 to in which it is shown applied to a P. A. X board.

Fig. 1 shows trunking arrangements of one of a number of identical units from which such a board may readily be built up, while Figs. 2, 3 and 4 respectively tabulate the relays required in the finder switch allotting equipment when the board comprises respectively one, two or three of the units shown in Fig. 1. It wil1 be seen from Figs. 2-4 that the relay allotting equipment per unit comprises but four relays where one unit only is concerned, the number of relays increasing slightly with increase in thenumber of units. These Figures 2, 3 and 4 are similar charts indicating between the dotted lines the relays which would be used in each allotting equipment and the reference characters used therein correspond to those of the relays in Figs. 7, 8 and 9. For instance, Fig. 2 shows that for a 35 line unit, 1 allotter including relays PR, FT, GR and UF would be required. Fig. 3 shows 2 allotting equipments between the 2 sets of dotted lines and shows the reference characters of the relays which would be required for a 35-70 line unit. Fig. 3 shows by dotted lines the 3 allotters which would be required in a larger system and indicates by reference characters the additional relays which would be used.

Figs. 5-10 when arranged in the manner shown in Fig. 11, which is located on the same sheet with Fig. 7, show circuit connections of a 100- line board which is built up from three 35-line identical units each comprising the subscribers line equipments, four finder/connector links and relay allotting equipment. In the circuits typical values of certain resistances and relay windings are shown.

Each link is assumed to employ a 50-point uniselector finder and a 100-point two-motion connector. Where the capacity of the board may exceed 50 lines, two line finders per link are provided and respectively serve subscribers in the under 50 and over 50 groups into which the 100 lines of the board are divided. Where the capacity of a board is not expected to need to exceed 50 lines, a 50-point finder per link will be provided.

A board for up to 35 lines will require one unit such as is shown in Fig. 1 comprising four links, such as Ul/Ll with associated finder and connector switches FS and CS, the subscribers line equipments such as LE, and also relay allotting equipment AEUI. In addition there will be a common sequence marking device or switch SMD which ensures cyclic operation of the relay allotting equipment at all times by successively marking each of the four links Ul/LlU|/L4 of the unit as the first choice link as will be fully described later.

A board with a capacity of from 35 to lines will comprise two units of the type shown in Similar remarks apply when the board involves three units when each of the three. allotting equip-.

ments will comprise the relays shown in Fig. 4. The common sequence marking arrangements are retained in each case. It will be clear from what has been said above that the subscribers linesof each unit are directly associated only with the allotter of that unit.

Where two or more units are concerned, the associated allotting equipments will normally operate independently in response to originated calls on the different units to give cyclic allotting of links. In case all links of a unit are engaged, calls from the lines of that unit will be transferred to the other unit or the next unit as the case may be, this being possible since the finders of the links of both or all units have access to all the lines of the board.

A P. A. X board employing a relay allotting circuit according to the invention is characterised by substantially instantaneous allotting of links or both units and links according to the prevailing circumstances. Moreove since the allotting equipments are on a per unit basis the total of allotting equipment will be proportional to the size of the board. Furthermore, as each of the allotters operates independently, simultaneous operation of two or more finder switches can take place provided they are in differing units.

As willbe appreciated from the ensuing'circuit description, link or unit and. link preselecting facilities are provided. As regards circuit operations, the line marking serves also for finder start purposes so that unless a bank marking exists a finder cannot be started up. Under blown fuse or other major fault conditions in a link, such link is automatically busied, while arrangements are also made for maintaining service under allotter or common sequence marking equipment failure. With regard to the sequence switch control of the allotting equipments, a selected link is adapted to operate for a predetermined period within which normally switching through takes place or the calling subscriber clears. If a finder fails to switch through before the predetermined period elapses, another finder in the same or another unit is caused to take up the search.

Referring now to Figs. 5 to 10, Fig. 5 shows a subscribers line equipment LE which, if the subscriber concerned is in the under 50 group, employs a line relay LU, while if he is in the over 50 group it employs a line relay LO, the contacts only ofwhich are shown. Figs. 5 and 6 together show the finder part of the finder/connector link Ul/Ll, which in common with the other three links Ul/LZ-UI/M is served by the allotting equipment AEUI part of which is shownin Figs. 7 and 8. The remaining two parts of; allotter AEUI which are connected with over leads T-Y are respectively identical with the corresponding parts ofthe allotters AEU2 and AEU3 which are shown in Figs. 9 and 10.

The circuit shows in Fig. 9 the particular part of allotter AEUZ to which a start signal would be transferred if a call on being initiated to almarking device SMD which serves allotter AEUI,

similar parts serving allotters AEUZ and AEU3. Each part comprises four sets of contacts over which wipers WI and W2 are continuously driven, conveniently from the shaft of the ringing current generator or from any other suitable source. These wipers are'adapted successively to earth for periods of the order of one second each the paired busying and testing leads such as BL! and TLI extending to the associated links.

Considering now the circuit operation, when the subscribers line SL, Fig. 5, assumed to be in the under 50 group is looped, relay LU operates. Assuming that this subscriber has no priority facilities, the strapping connections "0 individual to each line equipment will apply, in which case battery via resistance YB, Fig. 7, will extend via contacts PR3 and common lead 13 to mark the calling line in the under 50 finder switch bank U84 of each link on the board, while it will also extend via common lead 15, contacts PR2 and resistance YC to relay UF, and also over contacts UF2 and start transfer lead 30 to the first transfer relay UTA, Fig. 9, in alloter AEU2 and over contacts UTA2 to second transfer relay UTB in allotter AEU3, Fig. 10.

Assuming that at least one of the links such as UI/Ll, Figs. 5 and 6, in the calling first unit is free, the busying relay GR, Fig. 8, will be normal and relay UF will operate, thereupon at contacts UF2 opening the circuit to the second and third allotter relays UTA and UTB. At UF3 it disconnects any circuit for relay OF, at UF4 prepares the priority cut-in circuit, and at UF5 earths the Y lead extending to the wipers WI and W2 of the sequence marking device SMD.

It will be seen that the left-hand contacts of this device are shorter than the right-hand ones so that wiper W2 will make contact before WI, the period during which, wiper W2 only is making contact being of the order of 20 milli-seconds and constituting a testing period during which the FS relay in the link with which the test lead connects can be operated, After this period the link is automatically busied, as will be described, if the FS relay is not already operated, and a call coming in at this stage would be switched into the next available link. Assuming that at the time relay UP is operated the sequence switch wipers are occupying the position shown, earth will extend over test lead TLI to relay F6 in link UI/LI, via rectifier MRB and contacts FSB to the FS relay in Ul/L2 and so on for the remaining two link the four FS relays being in effect tapped off a circular chain of rectifiers on to which chain an earth is applied at a partcular point in accordance with the position of the sequence switch wipers. Under the circumstances being considered, the earth is applied directly and not via any FS relay contacts to the FS relay in link U1 /LI, and this relay will therefore have the first choice of the four FS relays the chain circuit, even though all the links Ul/L|U|/L4 may be idle. In this case all the FS relays will be connected to resistance battery as for instances via lead 23, Fig. 6, contacts GR3, lead 24, test springs TI, connector oil'- normal contacts NI, contacts SW4 and resistance YE to battery, for relay FS in link UI/LI. Relay FS will therefore operate and in so doing will disconnect any circuit to the other FS relays at contacts PS8.

In similar manner with the switch in the next position, link UI/LZ would be marked over lead TL2 as the first choice and so on, the rectifiers MRB, MRD, MRF and MRH functioning in each case to ensure that the circuit always operates in the predetermined clockwise direction,

Returning now to the operation of relay FS, at contacts FS5, Fig. 5, it locks relay UF over lead 2!] to the starting and marking battery and also energises relay GR, Fig. 8, which at its contacts GR3 disconnects the initial idle resistance marking battery of the selected link but this is without efiect owing to it being short-circuited by operated contacts FSl. Relay FS also at contacts FS2 prepares a self-interrupted driving circuit for the under 50 finder switch magnet USM via the magnet interrupter contacts USMC, and at contacts FS8 extends the test lead earth to operate relay A which brings up relay B, not shown, in known manner. Relay B completes the driving circuit of magnet USM at contacts B3 and at contacts B2 connects the allotter low resistance testing relay FT to wiper US I.

The sequence switch wiper WI will by this time have reached the first contact and both the leads BLI and TLI will be earthed so that relay FS would be short-circuited were it not for the fact that it has already operated and opened the short-circuitin path over the busying lead BLI at contacts FSI. Relay FS continues to hold over the earthed test lead TLI.

When the finder switch reaches the calling line, relay FT rapidly operates in series with the line marking battery and by virtue of its low resistance provides an immediate guard on this line. At contacts FTI it cuts the driving circuit and extends earth over common lead I9 to bring up relay SW in series with the calling subscribers cutoff relay K. Relay SW in operating, at contacts SWI and SW2 switches the calling line through to the A relay of the link via banks and wipers USI and USZ, at contacts SW3 extends a low resistance guarding earth through its upper low resistance winding to bring up relay K, at contacts SW4 disconnects the link idle resistance markin battery, whereupon relay FS releases, and at contacts SW5 extends the link busy indication lead 25 through to the next link UI/LZ.

Relay FS in releasing releases relays FT, UP and GR and relay K in operating releases relay LU which removes the starting and marking resistance battery. The allotter AEUI is now ready to deal with further calls from the group of subscribers which it serves.

In link UI/LI, relays A, B and SW remain operated and in response to dialled impulses the connector portion is subsequently set to eiTect connection with the wanted line via the connector switch multiple such as CSM in known manner.

From the foregoing description it will be seen that the whole of the finding operations described are dependent on the continued operation of the line relay LU so that if the calling subscriber hangs up his receiver at any time during finding, all apparatus will be released, the finder switch remaining in the position that it reaches when its driving circuit is cut ofi.

. Assuming that in the circumstances described a subscriber in the over 50 group in the first unit calls, relay OF, Fig. '7, operates from the YA resistance battery in place of relay UF from the YB resistance battery and functions in precisely the same manner as relay UF with the exception that when relay FS operates in the selected link UI/LI, relay CO is brought up over common lead I6 and at its contacts CO2 connects up the over 50 finder switch magnet OSM and at its other contacts connects up the corresponding wipers and banks. Relay CO locks up to contacts SW6 so that it will remain held for the duration of the call.

As previously mentioned each finder is allowed a predetermined period within which to efiect its switching through to a calling line, this period being governed by the sequence switch and being generally terminated when the wipers WI and W2 leave the pair of contacts associated with the selected link. The period concerned is at least such as to allow the finder switch to perform a complete revolution and is a minimum when the selected link is the first choice link. In the instance described, where the link UI/LI taken into use is the first choice link, if the finder switch fails to switch through by the time Wipers WI and W2 leave the contacts connecting with the associated leads BLI and TLI, earth will be removed from lead TLI and relay FS will thereupon release. Since wiper W2 is of the bridging type, at the same time as relay F8 in the link UI /LI is released, earth will be extended over test lead TL2 to bring up relay F5 in the next link III/L2 provided this is free or the relay FS in the next free link in the sequence. This link is started up in the same manner as described for the link UI/LI which is released on the release of relay FS.

In case link UI /LI with the faulty finder switch is the only available link in the first unit, then in order to ensure that this link is not repeatedly taken into use contacts CI, Fig. 6, of relay C (not shown) of the A, B and C impulse-responding relay triad in that link are provided in the link busy indication circuit. When link UI/LI is released, relay C in operating in usual manner during the release of relay B will maintain a circuit for relay GR, Fig. 8, for sufiicient time to ensure that a free link in another unit is taken into use via one of the transfer relays in a manner to be described.

Assuming that the under 50 group subscriber calls when the sequence switch wipers WI and W2 are connecting with leads BLI and TLI relay UF will operate as usual if any one of the links associated with allotter AEUI are free, but relay FS in first choice link UI/LI will be unable to operate since it will be short-circuited over leads BLI and TLI. If the next link UI /L2 is free, the relay FS in this link will therefore operate and will cause the associated finder to search for the calling line. In this instance it will be seen that the finder switch of link UI /I.-: will have a longer time to complete its switching since the circuit to the FS relay will not be opened by the bridging wiper W2 until such time as it leaves the contact connecting with the test lead TL2 for link UI/L2. In similar manner links UI/L3 and UI/L4 if taken into use in these circumstances would have even longer times to complete their finding operations.

Assuming now that the subscriber calls with the sequence switch in. the position shown, and that link UI/LI is busy. Under this condition the resistance battery connection to relay FS will be disconnected so that the earth over test lead" 7 TL! will extend via rectifier MRB to bring up relay FS in the next free link, whereupon the operations continue as before described.

Considering now the arrangements for start signal transfer under busy conditions, the under 50 and over 50 relays UF and OF, the first transfer relays UTA and OTA, and the second transfer relays UTB and OTB of each unit are interconnected over leads designated TY in each instance. Certain pairs of relays in each of the units are linked together as indicated in Fig. 3 and as shown in detail in Figs. 7-10 with particular regard to the linking between the relays UF and OF of the first unit, relays UTA and OTA of the second unit and relays UTB and OTB of the third unit.

For a call initiated on the first unit, if all the links therein are busy, the call is transferred to the second unit in the manner described below where relay UTA or relay OTA is brought in according as to the group in which the calling subscriber is situated. If all the links in the second unit are engaged, the call is transferred to the third unit where rela UTB or relay OTB is brought in. Similar remarks apply when a call is initiated on the other units. Thus each subscriber on the board has potential access to all links of the board.

Assuming now that an under 50 group subscriber in the first unit calls when all the associated links are busy, relay GR, Fig. 8, will already be operated via the series-connected contacts SW5 in the various links and this relay will have disconnected the earth connection to relay UF at contacts GRI. Hence the starting battery will be extended over lead 30 to the second unit to operate relay UTA, Fig. 9, which functions in similar manner to relay UF of the first unit, i. e. over lead Y, Fig. 9, it earths the sequence switch wipers associated with the second unit and so brings up the FS relay in a free one of the associated links, the first choice-link being determined in usual manner by the position of the sequence switch wipers.

If all links are busy in the second unit, relay GR of this unit will be operated and will have disconnected any circuit to relay UTA at contacts GR! so that the start battery will be effective on relay UTB, Fig. 10, in the third unit. Relay UTB in operating starts up a free link in the third unit.

In similar manner an over 50 call in the first unit would be transferred to the second or third units under heavy calling conditions, the relay concerned being relay OTA, Fig. 9, or OTB, Fig.

With regard to the function of the GR relay in each allotter this is operated as described over its upper winding under conditions where all associated links are busy, but, as mentioned earlier, it is also operated over its lower winding when any one of the four associated links is taken into use and is then held until the link completes its finding operation. If, for example, an under 50 subscriber in the first unit has taken link Ul/Ll into use, relays UP and GR will be operated in the allotter and relays FS, A and B will be operated in the link, while the finder switch will be commencing to search for a calling line. Relay GR in operating, at contacts GRd, GR5 and GRB (not shown) corresponding to GR3 disconnects the resistance battery from the FS relays of the other links in that unit. If the link Ul/Ll is not the first choice link at that time, in which case relay FS will be holding via contacts FSB of a prior link or links and the relevant chain circuit rectifier or rectifiers, and a prior link should become free, then if such link could become operative by i e-operating its FS relay, link Ul/LI in question would be broken down. Since, however, the resistance battery to the three FS relays of links Ul/LZ-Ul/Ll is disconnected by relay GR, this cannot take place and the normal operations proceed without interference. Similar conditions exist between units, but in this case interference is prevented in well-known manner by means of a low resistanc guarding earth through one winding of the switched-in relay concerned. For example, if a call on the first unit ha been transferred to the second unit, in which case relay UTA or relay OTA, Fig. 9, will be operated together with the FS relay in the link taken into use, a comparatively low resistance earth will be fed back via lead V and the upper low resistance winding of relay UTA or the lower low resistance winding of relay OTA to prevent relay UF or OF, Fig, 7, from switching if a link in the first unit becomes free immediately afterwards. In this manner a finder in an early unit is prevented from taking over a call from a finder in a later unit.

Relay GR in operating also at contacts GR! and GR& (as shown in Fig. 9 for allotter AEU2) disconnects the earth feed to the first transfer relays UTA and OTA and at the latter contacts applies earth to lead U, which is utilised when three allotters are employed on the board, to bring up a third relay GRR (shown in Fig. 10 for allotter AEU3) and this relay at contacts GRRI and GRRZ disconnects the earth feed from relays UI'B and OTB. It will thus be seen that when a link finder is searching, the operation of relay GR busies the controlling allotter against all further calls.

In case calls are originated simultaneously by two subscribers located respectively in the under 50 and over 50 sections in the same unit, either relay UF or relay OF will operate first and will disconnect the other so that the transfer lead 30 or 3| as the case may be, associated with the unoperated relay remains energised and the call for the second subscriber will mature in the next free unit.

In the case where subscribers in the three units originate calls simultaneously, the UP or OF relay in each unit will be operated so that it will be seen that under these circumstances three calls can be dealt with simultaneously, one in each unit.

Considering now the priority facilities afiorded to privileged subscribers, such subscribers are all located in the under 50 group so that they always appear on the banks of the under 50 finder switches US in the various links. The arrangements are such that a privileged subscriber on initiating a call is immediately allotted a free link and starts up the US finder switch in that link. At the same time the starting and marking condition for under 50 ordinary subscribers is disconnected so that there will be no possibility of a link finder switch, started up by an "under 50 ordinary subscriber, connecting with the privileged line or vice versa.

Considering now the circuit operations, it will be assumed that a privileged subscriber in the first unit is calling. In the line circuits of privileged subscribers the strapping connections P are used so that a starting potential for such subscriber will extend to common lead [4 and thence via rectifier MRJ to lead 32 which is common both to the rectifiers of the leads I4 of the three allotters of. the board and to the PR relays of the three allotters of the board, In consequence the PR relays in all three ,allotters will operate -In the calling allotter AEUI relay PR in operating at contacts PRI disconnects the starting condition for all over 50 subscribers in the first unit so that link finders already in operation to find such subscribers will be released. A link finder about to switch through to such subscriber will, however, be able to do so since the marking condition is not disconnected by relay PR. At contacts PR2 the starting battery for the privileged subscriber is extended via resistance YC to relay UP, and at contacts PR3 the starting and marking condition for all under 50 ordinary subscribers on the first unit is disconnected. If allotter AEUI is not already in use at this time, relay UF will operate and will start up a free link in search of the privileged party in the usual manner, all other markings on the bank of the link finder switch being disconnected in response tothe operation of the PR relays in the three allotters in the manner described for relay PR of the first unit.

If an ordinary under 50 call is already being handled by relay UF in the allotter AEUI at the time relay PR operates, relay UF will be maintained by relay PR and the link finder in operation will thus be taken over by the privileged party.

In case an over 50 subscriber and the privileged subscriber on the first unit originate calls simultaneously, contacts PR4 in the circuit of relay UF will give this relay priority over relay OF.

If an ordinary under 50 call is being already handled by relay UTA or UTB in allotter AEUI, relay PR in operating will operate relay UTA i: allotter AEUZ and thus start up a link in the second unit. The operated relay UTA or UTB in the first unit will be released until such time as the privileged party has been switched through to the ink.

If an ordinary over 50 call is being already handled by relay OF, OFA or OFB in allotter AEUI, relay PR will operate relay UTA in allotter AEU2 as before and relay OF, OFA or OPP, in allotter AEUi will be released.

Should priority subscribers in different units originate calls together, a finder switch will be started up in each of the units concerned, but if a plurality of priority subscribers in one unit originate calls together, only one finder switch in that unit will be started up. The rectifier such as MRJ in each of the allotters prevents the starting battery of one or more calling priority subscribers in one unit from extending through via the common lead 32 to the UP relays in the other units and so starting up a plurality of finders in search of one subscriber. The operation of the PR relays in the various units is not affected since only one rectifier in a conductive direction, rectifier MRJ in this instance, Will be in circuit with the PR relays, whereas two rectifiers in conductive and non-conductive directions respectively will be in circuit with the UP relays of other units.

From further examination of all possible prevailing conditions it will be seen that a privileged subscriber on calling will receive substantially im mediate priority over all other subscribers in process of being connected up to a free link.

It will be noted that when a finder is set in operation by a privileged party, battery is extended via resistance YD, Fig. 7, contacts UF4 or their equivalent on relay UTA or UTB, contacts PR5, lead 2| and contacts PS6 to operate relay D, Fig. 6, in the selected link so as to prepare the link manner.

1.0 circuit, for giving a priority cut-in feature to an engaged line when subsequently the link has been set to a desired line by dialling in the usual It will be understood that other special facilities may be provided in similar manner.

Although in the example described the privileged subscribers are assumed to be located in the under 50 group, it might be desirable to arrange for them to appear in either group, one necessary circuit modification with this point in view being to arrange for the PR relays in operating to disconnect both the markin andstarting condition for all ordinary subscribers on the board.

Considering now the operation of the board under fault conditions, if the two-motion connector switch in any link stays off-normal through some mechanical fault, the resistance battery to the FS relay will be disconnected at contacts NI, Fig. 6, while at contacts N2 a point will be completed inthe busy indication chain circuit. Similar remarks apply if the link is receiving attention by a maintenance oificer, in which case the U plug in test springs TI is transferred to springs T2. If a fuse associated with the battery feed to a link should blow, the resistance battery to the FS relay will be inefi'ective and since in such circumstances the battery connection to resistance YE will be replaced at the fuse bar by the fault earth which caused the fuse to blow, such earth potential is prevented from feeding back through relay FS by means of rectifier MRA and so is prevented from upsetting the link allotting chain circuit. If the sequence marking device should stop with its wipers in a position such as that shown, every link on the board would be able to give service, but if it should stop with both its wipers connecting with contacts, one link in each unit would be out of action owing to the FS relay being short-circuited over the leads with which the sequence switch wipers are connecting. Under the latter conditions, however, arrangements might be made whereby all the busying leads BLI-BL4 would be automatically or manually disconnected whenever the sequence switch failed, and hence the whole board could be maintained in action no matter in what position the sequence switch wipers are stopped, though clearly the cyclic allotting feature would be lost.

It will be understood that the invention is not limited in application to P. A. X boards as it could also be applied to P. A. B. X boards or any other boards employing line finder arrangements including main exchange line finder arrangements with or without partial secondary working.

What we claim as new and desire to secure by Letters Patent is:

1. In a telephone system, a group of lines, a group of finder switches having acces to said lines, an allotter comprising a chain of relays one for each finder, a closed chain circuit including said relays and a unidirectional current conductinng device in the circuit between each relay and the next, and a switching device operated to apply starting potential to each relay in turn when a call is initiated to allot an idle finder for use in handling the call.

2. In a telephone system, a group of lines, a group of finder switches having bank contacts connected to said lines and responsive to markings on said contacts to connect with said lines, means controlled from calling lines for placing said markings on said contacts, and means controlled from certain priority lines, when calling, for removing previously established markings 11 from said contacts to insure that the next operated finder will connect with the calling priority line.

3. A telephone system as claimed in claim 1 in which there are a pair of contacts of difierent length for each finder and the switching device comprises a continuously operated rotary switch having means for applying potential to said contacts so that for the one period potential is applied to one contact only and for another period to both contacts.

4. In a telephone system a group of finder switches having access to a group of lines, a relay allotter for allotting an idler finder to extend calls from calling lines comprising a closed chain circuit having a relay for each finder connected thereto and a one way conducting device in the chain between each relay and the next, a pair of contacts for each finder switch, a high speed r0- tary switch in the allotter for applying starting potential to each relay connection in turn, said switch operating at a speed such as to pass over each pair of contacts in slightly more time than that required for a finder switch to hunt over all of it contacts.

5. A telephone system as claimed in claim 4 in which the allotter relay for initiating the operation of the associated finder is connected across a pair of said contacts and is short circuited when potential is connected to both contacts by said switch.

6. A telephone system such as claimed in claim 1 in which each relay upon operation has means for cutting off the subsequent relays in the chain circuit.

JOHN WILLIAM MCCLEW. OWEN AVIS PEARCE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,849,694 Saunders Mar. 15, 1942 1,924,661 Stehlik Aug. 29, 1929 2,149,646 Voss Mar. 7, 1939 

